1. Field of the Invention
This invention relates to carbon nanotubes, and more particularly relates to a carbon nanotube attached with quantum dots through non-covalent bonding to become visualizable in an optical microscope, and to a method of visualizing a carbon nanotube.
2. Description of the Related Art
Since Iijima (Nature 1991, 354, 56) discovered carbon nanotubes (CNTs), the studies on the structures and electrical, mechanical and chemical properties thereof are much developed. However, for the size of CNT is in nanoscale, observation of CNT requires an expensive and operationally complicated instrument, such as atomic force microscope (AFM), transmission electron microscope (TEM) or scanning electron microscope (SEM). Meanwhile, the procedure of preparing a CNT specimen for the above microscopes is tedious. Moreover, using an electron microscope to observe CNTs tends to damage the structure thereof or even break them apart due to the high-energy electron beam, and is thus not an efficient and economical observation method.
Due to its structure and dimension, CNT has drawn much attention in the fields of biology and chemistry and has been utilized in the applications of medical micro-probes and sensors. However, for the applications are usually done in a wet condition, e.g., in a liquid solution, TEM or SEM is too sophisticated in use. The simplest and cheapest solution to the above issue is to use optical microscopes, while many optical observation methods for CNTs have been proposed recently. For example, a CNT can be attached with a fluorescent organic dye at its surface or covered by a fluorescent polymer so that it can be observed directly with a fluorescence optical microscope. However, the two methods have certain drawbacks. The luminescence lifetime of a fluorescent organic dye is quite short preventing long-term observation or labeling. Furthermore, the interaction between a fluorescent polymer and the surface charges of the CNT reduces the fluorescence of the polymer, or even eliminates the fluorescence to make the polymer lose its labeling function. To solve the issues, many recent studies focus on attaching quantum dots, instead of organic dyes or fluorescent polymers, to CNTs so that the CNTs can be easily labeled and observed in a wet condition.
A quantum dot is usually a semiconductor nanocrystal with a very long lifetime and a very high efficiency in the luminescence of which the luminescence color can be easily changed by adjusting the size thereof, thus being quite suitable for labeling. A quantum dot can be attached to a CNT through covalent or non-covalent bonding. Though forming a covalent bond firmly fixes the quantum dot on the surface of the CNT, the unique structure of the CNT has to be destroyed previously to form a dangling bond for linking the quantum dot, so that the unique properties of the CNT are affected. On the other hand, a non-covalent bonding method can retain the unique properties of a CNT, and can make the quantum dots uniformly attached to the CNT as not needing to form a covalent bond at a specific site on the CNT surface. However, the non-covalent bonding in the prior art is generally weaker making the quantum dots easily detached.